(1) Field of the Invention
The present invention relates to a resin composition excellent in resistance to thermal creep, resistance to water vapor permeation and resistance to impact, which is molded or formed into a battery case for secondary battery (including battery cases for enclosed secondary battery).
(2) Description of the Related Art
Secondary batteries (including enclosed secondary batteries) known to include lead storage batteries, nickel-cadmium batteries, nickel-hydrogen batteries and the like are widely used as power sources for vehicles (e.g. automobiles), various electric appliances and industrial machineries and tools. There has been a growing demand for them in recent years. With the growing demand, the batteries themselves have been improved in their performances, their battery cases have been miniaturized and lightened, and the electric capacity of the batteries has been increased. Therefore, there is a need for the replacing the material for the battery cases with resin materials which are resistant to change in shape, such as design variation and thinning and lightening. Moreover, in order to improve the battery performance, a resin material which has excellent mechanical strength, resistance to water vapor permeation and heat resistance is required.
In a battery case for secondary battery made of a resin, there have been used, for example, ABS resins and modified polyphenylene ether resins from the viewpoint of moldability and heat resistance. These resins, however, are poor in resistance to water vapor permeation and hence have a fatal defect in that water contained in an electrolytic solution in the secondary battery or a separator permeates the battery case for secondary battery during long-term use. As a result, the concentration of the electrolytic solution varies leading to the deterioration of battery performance.
In addition, the ABS resins and the modified polyphenylene ether resins are poor in chemical resistance. These materials, therefore, have a disadvantage in that they cannot be used for a battery case for secondary battery used in automobiles for a long period of time because of their low resistance to chemicals such as automotive oil.
In order to solve the above problems, JP-A-8-195188 proposes a battery case for enclosed secondary battery made of a resin composition comprising a crystalline polyolefin resin and a polyphenylene ether type resin. JP-A-9-120801 has proposed a battery case for enclosed secondary battery obtained by the use of a polymer alloy comprising a polyphenylene ether resin and a polyolefin resin. Re-publicized International Publication No. WO97/01600 discloses that a resin composition comprising a crystalline polyolefin resin and a polyphenylene ether resin, wherein the polyphenylene ether resin is dispersed in a specific form in a crystalline polypropylene resin, can be utilized as a material for a battery case for secondary battery because of its excellent resistance to thermal creep and resistance to water vapor permeation. JP-A-2000-58007 discloses that a resin composition comprising a polyphenylene ether type resin and a crystalline polypropylene having a specific structure can be utilized as a material for a battery case for enclosed secondary battery.
As to a resin composition for battery case which comprises a crystalline polypropylene type resin, JP-A-6-287364 discloses that the degree of shrinkage and deformation of a composition comprising a polypropylene type resin and talc with a specific particle size is low after its molding or forming. JP-A-8-132468 proposes a polypropylene battery case comprising a polypropylene resin and a slight amount of talc. JP-A-11-31483 proposes a battery case for enclosed electrochemical battery, produced from a composition having a crystallinity of 55 to 65% and comprising at least 95 wt % of a polypropylene and the balance of a propylene-ethylene copolymer. JP-A-2000-182571 proposes a material for battery case which comprises a blend of propylene, an α-olefin block copolymer and an ethylene/α-olefin copolymer and is excellent in the transparency.
These battery cases for secondary battery obtained from the resin compositions described above, however, contain a large proportion of one or more components other than a crystalline polypropylene which are added in the preparation of a polymer alloy. Therefore, it was difficult to maintain the excellent resistance to water vapor permeation of the crystalline polypropylene in the final resin composition at a high level.